Piezoelectric thin film resonators have been used for filters of wireless communication devices such as mobile phones. The piezoelectric thin film resonator includes a piezoelectric film, and a lower electrode and an upper electrode facing each other across the piezoelectric film. In recent years, to improve the performance of the wireless communication devices, it is desired to broaden the band of the filter characteristics. The band of the filter characteristics can be broadened by increasing the electromechanical coupling coefficient of a piezoelectric thin film resonator used in the filter. The electromechanical coupling coefficient of the piezoelectric thin film resonator can be increased by using a piezoelectric film with a large electromechanical coupling coefficient.
For example, it has been known that the electromechanical coupling coefficient can be increased by using an aluminum nitride film containing a II-group or XII-group element and a IV-group or V-group element for the piezoelectric film as disclosed in, for example, Japanese Patent Application Publication No. 2013-219743. In addition, it has been known to make the concentration of an additive element in an end region in the thickness direction of the piezoelectric film lower than that in a middle region, the piezoelectric film using an aluminum nitride film containing the additive element as disclosed in, for example, Japanese Patent Application Publication No. 2014-121025. It has been also known to use an aluminum nitride film containing scandium for the piezoelectric film as disclosed in, for example, Japanese Patent Application Publication No. 2013-128267.
Use of an aluminum nitride film containing a II-group or XII-group element and a IV-group or V-group element for the piezoelectric film increases the electromechanical coupling coefficient, but decreases the Q-value that is in a trade-off relationship with the electromechanical coupling coefficient